.\"	$OpenBSD: strncpy.3,v 1.2 2014/04/19 11:30:40 deraadt Exp $
.\"
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.\" Chris Torek and the American National Standards Committee X3,
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.Dd $Mdocdate: April 19 2014 $
.Dt STRNCPY 3
.Os
.Sh NAME
.Nm strncpy
.Nd copy part of a string to another
.Sh SYNOPSIS
.In string.h
.Ft char *
.Fn strncpy "char *dst" "const char *src" "size_t len"
.Sh DESCRIPTION
The
.Fn strncpy
function copies not more than
.Fa len
characters from the string
.Fa src
to the buffer
.Fa dst .
If
.Fa src
is less than
.Fa len
characters long,
it fills the remaining buffer with
.Ql \e0
characters.
If the length of
.Fa src
is greater than or equal to
.Fa len ,
.Fa dst
will
.Em not
be NUL-terminated.
.Pp
.Fn strncpy
.Em only
NUL terminates the destination string when the length of the source
string is less than the length parameter.
.Pp
If the
.Fa src
and
.Fa dst
strings overlap, the behavior is undefined.
.Sh RETURN VALUES
The
.Fn strncpy
function returns
.Fa dst .
.Sh EXAMPLES
The following sets
.Va chararray
to
.Dq abc\e0\e0\e0 :
.Bd -literal -offset indent
(void)strncpy(chararray, "abc", 6);
.Ed
.Pp
The following sets
.Va chararray
to
.Dq abcdef ,
without a NUL-terminator:
.Bd -literal -offset indent
(void)strncpy(chararray, "abcdefgh", 6);
.Ed
.Pp
The following sequence copies as many characters from
.Va input
to
.Va buf
as will fit, and then NUL terminates the result by hand:
.Bd -literal -offset indent
char buf[BUFSIZ];

(void)strncpy(buf, input, sizeof(buf) - 1);
buf[sizeof(buf) - 1] = '\e0';
.Ed
.Pp
By now it is clear that
.Nm strncpy
is dangerously easy to misuse.
The
.Xr strlcpy 3
function is safer for this kind of operation:
.Bd -literal -offset indent
if (strlcpy(buf, input, sizeof(buf)) >= sizeof(buf))
        goto toolong;
.Ed
.Sh SEE ALSO
.Xr strlcpy 3 ,
.Xr wcscpy 3 ,
.Xr wcslcpy 3
.Sh STANDARDS
The
.Fn strncpy
function conforms to
.St -ansiC .
.Sh HISTORY
The
.Fn strncpy
function first appeared in
.At v7 .
